1. FIELD OF THE INVENTION
This invention relates to a heat-resistant protective layer for a reversible optical information storage medium of a phase change type which can record, reproduce, erase and rewrite information with a laser beam.
2. DESCRIPTION OF RELATED ART
In an erasable, repeatedly recordable/reproducible and non-volatile optical information recording medium, for example, an optical disk memory including a phase change type or magneto-optical type one, a substrate is made of a plastics. In order to avoid the damage of the plastic substrate by heating with laser, it has been proposed to provide a heat-resistant protective film of an inorganic material such as oxides between the substrate and an optically active layer, on the optically active layer or on both the sides of the optically active layer (hereinafter referred to as the "active layer"). As materials for the heat-resistant protective layer are known oxides such as SiO.sub.2, GeO.sub.2, Al.sub.2 O.sub.3 and BeO.sub.2, nitrides such as BN, Si.sub.3 N.sub.4, AlN, carbides such as SiC, and chalcogenides such as ZnS and ZnSe. Main properties required for the heat-resistant protective layer are that
(1) the layer is transparent in a wavelength range used, PA1 (2) the melting point of the layer is higher than an operation temperature, PA1 (3) the layer has a high mechanical strength, PA1 (4) the layer is chemically stable, and PA1 (5) the layer has a suitable thermal constant such as thermal conductivity and specific heat.
The requirement (1) above is clearly necessary for the highly effective absorption of laser energy in the active layer; the requirement (2) is important because the heat-resistant layer is inconveniently changed before the active layer reaches the thermal transformation temperature, if the requirement (2) is not satisfied; the requirement (3) above is necessary to prevent the protective layer from being broken during the heating or cooling thereof; the requirement (4) is essential to avoid the hydrolyzation or deliquescence of the layer with moisture; and finally the lack of the requirement (5), i.e., an improper thermal constant causes the laser energy to be utilized with reduced efficiency, in the optical information recording medium, particularly the phase change type one which changes reversibly between the two phases thereof by quenching and annealing in combination for recording and erasing. Such an optical information medium having an improper thermal constant is insufficiently sensitive to laser irradiation power, that is, it requires higher laser irradiation power for recording and erasing.